Assessing the Importance of Nonlinearities in the Development of a Substructure Model for the Wind Turbine CAE Tool FAST

Abstract

The design and analysis of wind turbines are performed using aero-servo-elastic tools that account for the nonlinear coupling between aerodynamics, controls, and structural response. The NREL-developed computer-aided engineering (CAE) tool FAST also resolves the hydrodynamics of fixed-bottom structures and floating platforms for offshore wind applications. Primarily due to the required modal characteristics, monopiles become progressively less economical and more difficult (or impossible) to fabricate for multimegawatt turbines and water depths of more than 25–30 m. Derived from the oil and gas industry experience, light and stiff space-frame alternatives have been proposed to alleviate this problem. Lattice structures (e.g., jackets) are more complex to analyze and design than cantilevered monopiles, especially in terms of the structural dynamics of the coupled turbine-support structure system. This paper outlines the implementation of a structural-dynamics module (SubDyn) for offshore wind turbines with space-frame substructures into the current FAST framework, and in particular focuses on the initial assessment of the importance of structural nonlinearities. Nonlinear effects include: large displacements, axial shortening due to bending, cross-sectional transverse shear effects, etc. A nonlinear computational analysis is resource-intensive, thus it is important to assess the applicability of a linear approach to maintain high-fidelity results while still allowing for fast and efficient design simulations. Space-frame structural behavior can be controlled by a number of design parameters (e.g., member cross-sectional properties, number of legs, batter angles). Additionally, nonlinearities may manifest only at certain load levels. Several finite-element analyses were carried out via commercial and open-source codes that can capture nonlinear effects in the structural behavior of turbine substructures under different load cases. Results were compared to the output of the new linear module SubDyn. The configurations considered in this study included 5-MW, 7-MW, and 10-MW platforms: OC3 monopile, OC3 tripod, OC4 jacket, and a full-lattice tower, all supporting a 5-MW turbine; also two jackets for a 7-MW and a 10-MW turbine, respectively, were investigated. These models differed in base geometry, load paths, size, supported towers, and turbine masses. Results showed that nonlinearities (quantified in terms of the maximum differences in displacement and stresses with respect to a linear calculation) amounted to about 4% (3%) at tower top (at tower base), or about 10 cm (1 cm). This means that the absolute effects of nonlinearities are mostly associated with the tower. The linear approach used by the multimember structural module introduced in this paper was therefore deemed suitable to be utilized within FAST to analyze multimember substructures for offshore wind applications.